Power line system and method for the transmission of electric energy over low temperature electric conductors

ABSTRACT

In a power line system for transmitting electric energy over electric conductors cooled by a coolant flowing in conduits associated with the conductors which, in turn, are surrounded by a radiation shield cooled by a coolant flowing in conduits associated with the radiation shield; the flow rate of the coolant through any cross-sectional area laid across all the first-named conduits, as well as the flow rate of the coolant through any cross-sectional area laid across all the second-named conduits is unequalized. Further, the power line system includes refrigeration machines to set to the optimal operating temperatures the coolant associated with the conductors and the coolant associated with the radiation shield.

United States Patent Doose et a1.

[ 51 Oct. 10, 1972 [72] Inventors: Conrad Doose; Wolfgang Sassin,

both of .lulich, Germany [73] Assignee: Kernforschungsanlage JulichGesellschaft mit beschrankter I-Iaftung, Julich, Germany 22 Filed: Dec.15, 1970 211 App1.No.: 98,231

[30] Foreign Application Priority Data Dec. 15, 1969 Germany ..P 19 62745.3

[52] US. Cl. ..174/15 C, 62/97, 174/D1G. 6 [51] Int. Cl. ..H0lb 7/34[58] Field of Search....l74/15 R, 15 C, 16 B, DIG. 6, 174/99 B; 69/97,120, 122; 165/143, 144,

3,511,919 5/1970 Miller ..174/15 C 3,463,869 8/1969 Cooley et a1...174/15 C 3,406,245 10/1968 Watkins ..174/ 15 C 3,363,046 I/ 1968Endacott ..174/ 15 C 3,396,551 8/ 1968 Dimentberg ..174/ 15 C X3,343,035 9/ 1967 Garwin ..l74/DIG. 6

FOREIGN PATENTS OR APPLICATIONS 1,167,054 10/1969 Great Britain ..174/15 C Primary Examiner-Lewis H. Myers Assistant Examiner-A. T. GrimleyAtt0rney-Edwin E. Greigg [57] ABSTRACT In a power line system fortransmitting electric energy over electric conductors cooled by acoolant flowing in conduits associated with the conductors which, inturn, are surrounded by a radiation shield cooled by a coolant flowingin conduits associated with the radiation shield; the flow rate of thecoolant through any cross-sectional area laid across all the first-namedconduits, as well as the flow rate of the coolant through anycross-sectional area laid across all the secondnamed conduits isunequalized. Further, the power line system includes refrigerationmachines to set to the optimal operating temperatures the coolantassociated with the conductors and the coolant associated with theradiation shield.

6 Claims, 4 Drawing Figures PATENTEDom 10 m2 SHEET 2 [IF 4 POWER LINESYSTEM AND METHOD FOR THE TRANSMISSION OF ELECTRIC ENERGY OVER LOWTEMPERATURE ELECTRIC CONDUCTORS BACKGROUND OF THE INVENTION Thisinvention relates to an electric power line system of the type whereinthe electric conductors are disposed in or on tubular conduits throughwhich a coolant passes, and which are arranged in a tubular l vacuumcasing. The conductors are surrounded by a radiation shield which iscooled by one or more tubular conduits. In such a power line system theelectric conductors are cooled to very low temperatures (including thecondition of superconductivity) by means of refrigerating machines eachformed of a compressor, an expansion device, and heat exchanger means.The annular space defined by the tubular casing and the radiation shieldis fully or partially filled with a heat reflecting insulating material.

There are known power line systems in which the electric forward andreturn conductors are arranged in a common vacuum casing. There are alsoknown power line systems in which the individual electric conductorsextend, together with a sole associated coolant conduit in vacuumcasings which are separated from one another. In such power line systemsthe coolant streams are directed in such a manner that during normaloperation, for each cross-sectional area of the power line, thefollowing condition is met: The mass flow of coolant passing throughsuch cross-sectional area in one direction is identical to the mass flowof coolant passing through the same cross-sectional area in the oppositedirection. In case several coolants are used, this condition isfulfilled for each coolant separately.

Thus, for each coolant in each power line portion, at least one forwardflow conduit and one return flow conduit is used. In power line systemsof this type it is endeavored to optimali'ze the cooling of the electricconductors and the radiation shield; that is, to reduce the heateffects. Also, the flow looses have to be reduced to a minimum value. Afurther characteristic of known low temperature power cables is the useof one or more cold radiation shields which are cooled with at least onecoolant other than the coolant used for cooling the electric conductors.For this purpose at least one additional coolant circuit is provided.While, for example, for the cooling of the electric conductorspreferably helium is used, for the cooling of the radiation shieldliquid hydrogen or liquid nitrogen may find application.

In power line systems known heretofore it is very disadvantageous thatthe coolant quantities flowing in the tubular conduits in the forwardand in the return direction have to be identical and for each flowdirection there have to be provided tubularconduits which are thermallyinsulated from one another. ln this manner the number of coolantconduits is doubled, the diameters of the vacuum casing and theradiation shield, as well as the cold surfaces of the power line systemand the number of the required, thermally insulating supports increasevery significantly. The use of several refrigeration systems has furtherthe disadvantage that none of the radiation shields is operated at itsoptimal temperature, but only in a narrow temperature range which isdetermined by the thermodynamic properties of the liquid gas used. Incase of radiation shields where liquid H, and/or liquid N, is used, theoperating temperature is in a narrow temperature rang of about 20 K and77 K, respectively, whereas the cooling of these radiation shields wouldbe optimal in a range from 30 to 60 K.

In case a radiation shield is used that is cooled with liquid hydrogenand/0r liquid nitrogen, there is a further disadvantage in that for theradiation shields a plurality of coaxially arranged coolant conduits arenecessary, contributing to the structural complexity of the power linesystem. This leads to difficulties during the installation and therepair of the power line system. Besides, a power line system of theaforenoted type is prone to malfunctioning. Also, attempts have beenmade to provide power line systems in which the electric conductors aremaintained at low temperatures and in which one electric conductor,combined with the forward coolant conduit and the other electricconductor, combined with a separate-return coolant conduit, are arrangedin such a manner that each conduit is surrounded by a separate vacuumcasing and installed as an independent cable. Although in this mannereven in case of a three-phase current each individual strand may beconstructed in a relatively simple manner, in such a structure, however,the costs for the required vacuum casings and installation are doubledor tripled. In addition, the costs for providing the cooling alsoincrease in a very substantial extent because of the increase in thecold surfaces. Strong electric and magnetic fields which, because of thelarge lateral distances of the individual current conductors, do notcancel out by themselves, necessitate additional measures to maintain,for example, in case of transmitting alternating current, the eddycurrent losses at a permissible level.

If, on the other hand, the electric forward and return conductors, aswell as the forward and return coolant conduits, are arranged in acommon vacuum casing, as it is the case in other known power linesystems, one part of the precedingly discussed disadvantages iseliminated. In such arrangements, however, other disadvantages appear,such as a very complicated structure and the unsuitability of theindividual components for mass production. In known power line systemsof this type a great number of tubular conduits and electric conductorshave to be crowded in a narrow space in such a manner that a goodelectric and thermal insulation is nevertheless ensured even underextreme temperature changes and alternating mechanical stresses.

The complicated spacers and guides increase the heat effects and theflow losses of the coolants. Both factors necessitate an increase in thenumber of refrigerating machines and an increase in the required coolingoutput. Furthermore, the manufacture and installation of such a powerline system was found to be very difficult. Further, it is impossible toperform repairs, such as an elimination of a leak in the vacuum system,without causing consequential disturbances by such intervention.

OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention toprovide an improved power line system for the transmission of electricenergy over low temperature conductors (including the state ofsuperconductivity) permitting a heat transmission and thus the coolingof the tubular conduits combined with or containing the electricconductors and the cooling of the tubular conduits connected with one ormore radiation shields in a heat conducting manner by means of a singlerefrigerating machine and a single coolant, whereby the conduit systemfor the coolant has a substantially simplified and hence more economicalstructure.

It is a further object of the invention to provide an improved powerline system of the aforenoted type wherein the flows of coolant for thecooling of the electric conductors as well as the flows of coolant forthe cooling of the radiation shields may be optimalized independentlyfrom one another as a function of operating temperatures, coolant flowrate and flow direction, whereby simultaneously the cold surfaces andthe heat effects are reduced and, by shortening of the flow paths, theflow losses are diminished.

Briefly stated, according to the invention, in a power line system ofthe aforeoutlined type, the flow rate of the coolant for cooling theconduits associated with the electric conductors, as well as the flowrate of coolant for cooling the conduits associated with the radiationshields, are not equalized when viewed individually at a desiredcross-sectional area passing through the totality of the tubularconduits for the cooling of the electric conductors and through thetotality of the conduits for the cooling of the radiation shield. Insuch power line systems known structural components and equipment may beused. One part of the coolant flow is, after passing through at leastone portion of the power line system for cooling the electricconductors, increased to the temperature necessary to cool the radiationshield and/or one part of the coolant flow is, after passing through atleast one portion of the power line system for cooling the radiationshield, decreased to the temperature necessary for the cooling of theelectric conductors. In this manner it is achieved that the forwardand/or return transmission of each partial coolant stream is effectedunder changed thermodynamic conditions. In both cases the operatingtemperatures will be optimal; i.e., they are elevated to an optimalvalue in conduits associated with the radiation shield and they arelowered to an optimal value in conduits associated with the electricconductors.

ADVANTAGES AND GENERAL DESCRIPTION OF EMBODIMENTS AND CHARACTERISTICS OFTHE INVENTION According to an embodiment of the invention, therefrigerating machines are so connected in the coolant conduit systemthat a partial flow of the coolant emerging from a refrigerating machineenters all the conduits associated with or containing the electricconductors and other part of the same stream enters all conduits thatare in heat-conducting relationship with the radiation shield. All theseconduits form part of that portion of the power line system which joinan adjacent refrigerating machine in the direction of planned flow.Subsequent to passing through the aforenoted power line portion, bothpartial coolant flows enter the lastnamed adjacent refrigerating machinein the direction of flow.

- According to another embodiment of the invention, the refrigeratingmachines are so connected in the coolant conduit system that the coolantemerging in partial flows enters both a part of the tubular conduitsassociated with or containing the electric conductors and a part of thetubular conduits in a heatconducting relationship with the radiationshield. The coolant enters the conduits at those ends of conduit systemportions which join a refrigerating machine. Therefrom the coolant flowsto the adjoining refrigerating machines connected with the other ends ofconduit system portions. At the same time, one portion of the coolantemerging from the adjacent refrigerating machine enters as a partialflow, both the remainder of the tubular conduits associated with orcontaining the electric conductors and the remainder of the tubularconduits which are in heat-conducting relationship with the radiationshield. Thus, the last-named partial flows return into the refrigeratingmachine from which the first-named partial flows emerge and they do soas a counterflow with respect to these first-named partial flows.

Dependent on the prevailing conditions, it may be expedient to arrangethe beginning and the end of the power line system in such a manner thatthey coincide and thus a closed loop system is formed wherein thecoolant flow passes through the conduit systembetween adjacentrefrigerating machines without additional return flow conduits. Such anarrangement is advantageous in that the number of the necessary tubularconduits may be lowered to a minimum, thus obtaining a particularlysimple structure. It is a further advantage of this arrangement that thedirection of flow of the coolant in the radiation shield and in theconduits containing the electric conductors may be freely selected ifthe power line system is installed as a closed loop system. Thus, incase of a disturbance for example, in case of a breakdown of a portionof the closed loop system or in case of an increase in the outputrequirement accompanied by a requirement for an increase in the coolantquantities for a conduit portion between the refrigerating machines, apartial return of the conduit is possible under changed thermodynamicconditions adapted to the requirements and thus the remaining portion ofthe conduit system may continue its operation in the desired manner.

Instead of arranging the power line system as a closed loop system itmay be built in such a manner that the beginning and the end thereof areat a distance from one another. In such a case the refrigeratingmachines are so connected to the coolant conduit system that betweenadjacent refrigerating machines circulating flows are formed with thepartial flows emerging from and entering the refrigerating machines andflowing through those portions of the conduit system that interconnectthe refrigerating machines. The coolant, entirely or partially, flowswith the required cooling temperatures through the conduits associatedwith or containing the electric conductors and through the conduitsassociated with the radiation shields. In this manner the coolant passesthrough the conduits containing the electric conductors and the conduitsconnected in a heat-conducting manner with the radiation shield,preponderantly in a counterflow. This is advantageous because betweeneach adjacent refrigerating machine circuits may be formed with thepartial streams that flow in the conduits interconnecting suchrefrigerating machines and emerging therefrom and entering thereinto.Such circuits may be formed as a partial or complete forward and/orreturn flow of the coolant and under accordingly changed thermodynamicconditions.

The partial flows of the coolant emerging from the refrigeratingmachines are in a counterflowing heatexchange in different temperatureranges with the partial flows entering the refrigerating machines afterpassing through the corresponding conduit sections. The conduit whichpertain to the individual power line portions and which cool theelectric conductors and which, for cooling the radiation shield, are ina heat conducting relationship withthe latter, serve for the forward, aswell as for the return delivery for the coolant between two adjacentrefrigerating machines. The conduits are connected with therefrigerating machines in such a manner that the outflowing coolantenters the tubular conduits divided into partial streams of differenttemperatures and the coolant emerging from the tubular conduits entersthe refrigerating machine as a combination of partial flows of differenttemperatures.

The distance between the refrigerating machines is determined as afunction of the heat generated in the electric conductors, other heateffects, flow losses, as well as the pressure drop in the conduitsthrough which the coolant flows and which serve for the cooling of theelectric conductors and radiation shields.

it has already been proposed to arrange along a linear power line systemfor the transmission of electric energy several refrigerating machinesand to use the tubular conduits containing the electric conductors asforward and return coolant conduits between adjacent refrigeratingmachines. Such systems are described, for example, by D. A. Swift inPaper No. 1.46 of the 12th International Congress of Refrigeration heldin Madrid in 1967 and by an article starting on page 238 in the August1968 issue of Cryogenics. The coolant emerging from the refrigeratingmachine is divided into two partial streams and passes through theconduits of the two adjoining conduit portions. Thereupon the partialstreams are submitted to a cooling process in the adjacent refrigeratingmachines and subsequent to their exit from these two refrigeratingmachines, they are in the same manner divided and returned to the firstrefrigerating machine. The metallic cooling tubes, which form theelectric conductors, are used between two adjacent refrigeratingmachines as forward and return conduits. The flow rate through anycross-sectional area of the tubular conduits is thus equalized and theyhave the same operating temperature. The conduit cross sections are sochosen that in case of an identical flow rate through the forward andreturn conduits, between the coolant flow emerging from the firstrefrigerating machine and the coolant flow entering the respectiveadjoining refrigerating machines, identical temperature differencesprevail. in such a system solely the electric conductors are cooledwhile for the cooling of the radiation shield a separate cooling systemhas to be provided which operates at a higher temperature level and isassociated with separate refrigeration machines. Consequently, the powerline system precedingly described substantially has the samedisadvantages as the power line systems known heretofore, while thepower line system according to the invention has the advantage that forthe cooling of the electric conductors and the radiation shield only asole coolant circuit is necessary.

A particularly advantageous arrangement of the coolant conduits of thepower line system is provided if one part of the coolant leaves one ofthe refrigerating machines with a temperature T, while the other portionemerges therefrom with a temperature T (which is higher than T,). Thepartial stream having temperature T, enters, subsequent to cooling theconduits associated with the electric conductors, with a temperature T(which is higher than or equal to T the refrigerating machine which isnext in the direction of flow. The partial stream having a temperatureT;, enters, subsequent to cooling the radiation shield, with atemperature T, (which is higher than or equal to T the refrigerationplant next in the direction of flow. In the heat exchangers of thisrefrigerating machine there occurs, between the partial streams of thecoolant emerging from the refrigerating machine, a heat exchange in sucha manner that the entering partial streams are combined only after thepartial stream having the lower temperature is in a counterflow with thecorresponding emerging partial stream, heated to approximately T Thepower line system according to the invention thus has the advantage thatthe mass flow passing in one direction through any cross-sectional areaof the tubular conduits for the electric conductors and the tubularconduits for the radiation shield does not have to be the same as theoppositely directed mass flow; rather, the mass flows may be adapted tothe operational requirement for optimalizing the cooling.

It is further achieved that for the heat transmission in the entiresystem only a single coolant is used and further, between two adjacentportions of the power line system only a single refrigerating machinehas to be installed. Also, the operating temperatures of the partialstreams emerging from the refrigerating machines an having differentcooling functions (that is, the cooling of one or more radiation shieldsand the cooling of the tubular conduits containing the electricconductors), may always be selected at an optimal value. In case thepower line system is of the type in which the beginning and the end donot coincide, at least one partial stream is directed in counterflowwith respect to the other partial streams in the conduit system.

It is a further advantage of the invention that dependent uponrequirements, the power line system may be composed of coaxially oraxially parallel disposed conduits. Or, the arrangement of the conduitsin the system may be a mixture of the two. For this purpose only aminimum number of tubular conduits are necessary, so that the power linesystem according to the invention may be made in an economic manner.Also, because of the small number of tubular conduits and structuralelements, the risk of malfunctioning is significantly reduced. Thethermal insulation between forward and return coolant conduits, whichhas been a necessary component in power line systems known heretofore inorder to cool long cable portions from the ambient temperatures to thelow operating temperatures, may now be largely omitted.

If in the coolant conduit in the one direction the flow passage sectionis the same as for the forward'and return conduit combined, then, in thecase of equal absolute mass flow rate, that is, under identical coolingcapacity, the flow resistance is reduced because of the reduction in theinner surfaces and further, there is a reduction in the frictional heatgenerated by the flow resistance and also a reduction in the pressuredrop per unit length. The decrease of these magnitudes permits to coollonger cable portions with smaller refrigerating machines wherebyinvestment and operational costs are lowered with respect to cableshaving forward and return conduits for the coolant. Because of theomission, particularly in a closed loop conduit system, of theinsulation between forward and return coolant conduits and because ofthe simpler geometrical arrangement of the tubular conduits, a decreasein the cold surfaces is obtained, resulting in a decrease of heateffects. Thus, in addition to a decrease in flow losses, therefrigeration output may also be lowered.

The invention will be better understood from the ensuing specificationof several embodiments for practicing the invention, taken inconjunction with the drawmg.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic view of a closedloop system for practicing the invention;

FIG. 2 is a schematic view along line llII of FIG. 1;

FIG. 3 is a schematic view of a power line system with partial orcomplete coolant return flow through the conduits containing theelectric conductors; and

FIG. 4 is a schematic representation of a power line system with partialor complete coolant return flow through conduits in a heat-conductingrelationship with the radiation shield associated with the conduits.

DETAILED DESCRIPTION OF THE EMBODIMENTS For simplifying the illustrationof the power line systems according to the invention, the vacuum casingis not illustrated in the figures. Furthermore, the radiation shields,which are in general disposed between the cooled radiation shield 1 andthe vacuum casing and which consist preferably of highly insulatingmaterial as well as the devices known per se for heating or cooling thepartial coolant streams to be transmitted under changed thermodynamicconditions, are also not shown.

As seen in the figures, the coolant flows through the refrigeratingmachines 2 disposed at predetermined distances. The principal componentsof each refrigerating machine are the heat exchangers 3 and 4, anexpansiondevice (not shown in the figures) and a compressor 5. Thecoolant is pressurized in compressors 5 and thereafter, prior to beingadmitted to the tubular conduits 6 and 7, passes through the heatexchangers 3 and 4, and the expansion stages of the expansion devices inwhich it is cooled. In the embodiments shown in the figures, only apartial stream of the coolant emerging from the refrigerating machine iscooled to the very low temperature T,. This partial stream flows throughthe tubular conduits 7 with the temperature T for cooling the electricconductors 8. It is to be understood that it is feasible to combine theseveral tubular conduits 7 into a single coolant conduit.

After the coolant has passed through a conduit portion between tworefrigerating machines, it enters in the direction of flow with atemperature T, into the adjacent refrigerating machine 2 and passestherein through heat exchangers 4 and 3. At the same time, the coolantis drawn by the compressor 5 of the refrigerating machine 2 and thewarmer coolant flowing from the compressor 5 is cooled by means of thecolder coolant entering the refrigerating machine. For the cooling ofradiation shield 1, between the heat exchangers 3 and 4 the otherpartial stream of the coolant is branched off at a temperature T andintroduced into the conduits 6 which are in a heat-conductingrelationship with the radiation shield l. The several tubular conduits 6may be also combined into a sole tubular conduit.

In the embodiment according to FIGS. 1 and 2, the partial stream flowsthrough the conduits 6 in the direction of the refrigerating machine 2through which simultaneously passes the partial stream for serving thetubular conduits 7. The partial stream admitted to a refrigerationapparatus from the conduits 6 enters as seen in the drawing between theheat exchangers 4 and 3 into the heat exchanger 3 with a temperature T,.At the same time, it is combined with the partial stream which isadmitted to the refrigerating machine from the conduits 7.

In contradistinction to the flow directions of the coolants shown inFIGS. 1 and 2, these coolants may be transmitted in mutually oppositedirections in the conduits for the cooling of the radiation shield, andin the conduits for the cooling of the electric conductors.

In case a partial return delivery of the coolant is to be effected as itis advantageous in some cases then as seen in FIG. 3 the partial streamof the coolant emerging from the heat exchanger 4 with temperature T, isdivided in such a manner that at least one of the tubular conduits 7contains a coolant flow in an opposite direction. It is to be understoodthat by means of the conduit system according to the invention,arrangements for the coolant delivery are possible in which the at leastpartial coolant return delivery occurs at a temperature which is higherthan that necessary for cooling the electric conductors duringoperation. This may be achieved as shown in the embodiment according toFIG. 4 by providing that of the two or more conduits 6 connected withthe radiation shield l, in at least one there is an opposite coolantflow. For this purpose the refrigerating machines 2 of the power linesystem are arranged according to the invention in such a manner that thecoolant quantities passing through the refrigeration apparatus from theadjacent conduit portions with temperatures T and T are so selected thatthe inflowing and outflowing coolant quantities for each heat-exchanger3 are of equal magnitude. Thereby it is achieved that the refrigeratingmachines 2, despite the difference between the quantities of the coolantadmitted to the conduits 6 for the cooling of the radiation shield l andthe conduits 7 for the cooling of the electric conductors 8, may beoperated in a very economical manner.

That which is claimed is:

l. A method of circulating coolant in a power line system whereinelectric energy is transmitted over cooled electric conductors, saidpower line system includes coolant conduits associated with saidelectric conductors for cooling the latter and coolant conduitsassociated with a radiation shield for cooling the latter, a vacuumcasing containing said conduits, said electric conductors and saidradiation shield, with said power line system there are associatedrefrigerating machines having compressor means, expansion means and heatexchanger means, comprising the steps of 10 i tions of the power linesystem; the partial streams of the last-named step (B) return in acounterflow with respect to the partial streams of the lastnamed step(A) to the refrigerating machine from A. maintaining unequalized theflow rate of the coowhich emerge the last-named partial streams.

lant through any cross-sectional area laid across 4. A method as definedin claim 2, directing the coothe totality of the conduits for coolingthe electric lant, with the absence of return flow, to successiveconductors, downstream refrigerating machines in a closed loop B.maintaining unequalized the flow rate of the coopower line system.

lant through any cross-sectional area laid across 5. A method as definedin claim 3, including the folthe totalit of the conduits for coolin theradialowin ste tion shieldy g A g pr idmg, between ad acentrefrigerating C. increasing to the temperature value necessary formachmes, a flow of the f coolant the cooling said radiation shield, thetemperature Streams passmg f those pomons of the of a partial stream ofthe coolant subsequent to its power System wh'ch extefld and frompassing through at least one portion of the power refrigerating machinesand which interconnect the line system for cooling the electricconductors, p? and and B. giving the coolant flowing through conduitsas- D. decreasing to the temperature value necessary for socated Y theelecmc and h the cooling said electric conductors, the temperaflPwmgthru3h 4 assocliited wlth the ture of a partial stream of the coolantsubsequent radlatfon the Pperatmg m to its passing through at least oneportion of the tures in the refrigerating machines for cooling the powerline system for cooling the radiation shield. elecmc conductors the f fi h f l 2. A method as defined in claim 1, including the step A -P asdefined clam me udmgt e o of admitting a partial stream of a coolantemerging 10mg P l h from a refrigerating machine to all those conduitsasadmmmg afirst pamal coo K a F sociated with the electric conductorsand to all those Peramre from l of refngeratmg, machmes conduitsassociated with the radiation shield which i condults assoclated wlththe elecmc conduc formoa portion of said power line system that joins arefrigerating machine in the planned direction of cooadmmng a 5 coolantg'," 3 lant flow and admitting both last-named partial streams 3 t 3 1:119 29 to the downstream successive refrigerating machine m q f g f j yt L after flowing through said last-named portion of the she temperamrelg an t e power line system. tempgrimue i 1 3. A method as defined inclaim 1, including the foladmmmg Sal rst Pam coo am stream Y a lowingsteps: temperature T, to the downstream ad acent A. admitting thecoolant, emerging as partial refngeraung machfne Subs,equent t l streamsboth to one part of those conduits through the conduits associated withthe electric sociated with the electric conductors and to oneconductors; the emperatum h'gher than the part of those conduitsassociated with the radiation 40 tempFrzfmlre T shield which form theends of portions of said D. admitting said second partial coolant streamwith power line system; said ends joining a refrigerating a ?mpel',atureT4 the downstream f m a chine refrigerating machine subsequent to itspass ng 8 SubSeq"1enfly directing Said coolant to those through theconduits assoc ated with the radiation downstream adjacent refrigeratingmachines 51mm; temperature T4 hgher than the which are connected withthe other ends of said E gf F h h rfl th last-named portions of saidpower line system; and "f m a cat 3?? count; ow i C. admitting a partialstream of the coolant, emerg- P3111131 q e if exc dangers 0 ing inpartial streams from the adjacent refrigeratthe e rigeratmg mac me, aning machines, both to the remaining part of those combmmg the partialcoolant Streams conduits associated with the electric conductors afterthe g l stream warmed to a and to the remaining part of those conduitsas- Peramre stem sociated with the radiation shield which form por-

1. A method of circulating coolant in a power line system whereinelectric energy is transmitted over cooled electric conductors, saidpower line system includes coolant conduits associated with saidelectric conductors for cooling the latter and coolant conduitsassociated with a radiation shield for cooling the latter, a vacuumcasing containing said conduits, said electric conductors and saidradiation shield, with said power line system there are associatedrefrigerating machines having compressor means, expansion means and heatexchanger means, comprising the steps of A. maintaining unequalized theflow rate of the coolant through any cross-sectional area laid acrossthe totality of the conduits for cooling the electric conductors, B.maintaining unequalized the flow rate of the coolant through anycross-sectional area laid across the totality of the conduits forcooling the radiation shield, C. increasing to the temperature valuenecessary for the cooling said radiation shield, the temperature of apartial stream of the coolant subsequent to its passing through at leastone portion of the power line system for cooling the electricconductors, and D. decreasing to the temperature value necessary for thecooling said electric conductors, the temperature of a partial stream ofthe coolant subsequent to its passing through at least one portion ofthe power line system for cooling the radiation shield.
 2. A method asdefined in claim 1, including the step of admitting a partial stream ofa coolant emerging from a refrigerating machine to all those conduitsassociated with the electric conductors and to all those conduitsassociated with the radiation shield which form a portion of said powerline system that joins a refrigerating machine in the planned directionof coolant flow and admitting both last-named partial streams to thedownstream successive refrigerating machine after flowing through saidlast-named portion of the power line system.
 3. A method as defined inclaim 1, including the following steps: A. admitting the coolant,emerging as partial streams, both to one part of those conduitsassociated with the electric conductors and to one part of thoseconduits associated with the radiation shield which form the ends ofportions of said power line system; said ends joining a refrigeratingmachine, B. subsequently directing said coolant to those downstreamadjacent refrigerating machines which are connected with the other endsof said last-named portions of said power line system; and C. admittinga partial stream of the coolant, emerging in partial streams from theadjacent refrigerating machines, both to the remaining part of thoseconduits associated with the electric conductors and to the remainingpart of those conduits associated with the radiation shield which formportions of the power line system; the partial streams of the last-namedstep (B) return in a counterflow with respect to the partial streams ofthe last-named step (A) to the refrigerating machine from which emergethe last-named partial streams.
 4. A method as defined in claim 2,directing the coolant, with the absence of return flow, to successivedownstream refrigerating machines in a closed loop power line system. 5.A method as defined in claim 3, including the following steps: A.providing, between adjacent refrigerating machines, a circulating flowof the partial coolant streams passing through those portions of thepower line system which extend to and from the refrigerating machinesand which interconnecT the same and B. giving the coolant flowingthrough conduits associated with the electric conductors and the coolantflowing through conduits associated with the radiation shield therequired operating temperatures in the refrigerating machines forcooling the electric conductors and the radiation shield.
 6. A method asdefined in claim 1, including the following steps: A. admitting a firstpartial coolant stream with a temperature T1 from one of therefrigerating machines into conduits associated with the electricconductors, B. admitting a second partial coolant stream with atemperature T3 from the same refrigerating machine into conduitsassociated with the radiation shield; the temperature T3 is higher thanthe temperature T1, C. admitting said first partial coolant stream witha temperature T2 to the downstream adjacent refrigerating machinesubsequent to its passing through the conduits associated with theelectric conductors; the temperature T2 is higher than the temperatureT1, D. admitting said second partial coolant stream with a temperatureT4 to the downstream adjacent refrigerating machine subsequent to itspassing through the conduits associated with the radiation shield; thetemperature T4 is higher than the temperature T3, E. directing in a heatexchanging counterflow the partial coolant streams in the heatexchangers of the last-named refrigerating machine, and F. combining theentering, partial coolant streams after the colder partial stream iswarmed to a temperature T4 in step (E).